There are many processes available for the purpose of impregnating a preform with liquid resin in order to make a composite. These processes may be broadly characterized into two categories. One category includes wet lay-up methods while the other category features some form of resin infusion such as resin transfer molding (RTM).
Wet lay-up processes tend to have lower average quality than resin infused parts. They are labor intensive to manufacture because each layer of preform material must be individually coated with resin and carefully positioned by hand. One frequent problem with wet lay-up methods is air bubble entrapment inside the composite while the resin is being applied to the preform. A second difficulty is achieving a uniform part thickness and fiber volume fraction which ultimately influence the material properties. Another problem with wet lay-up processes is the excessive amount of fumes given off by the exposed resin before and during cure.
Resin infusion methods are carried out in a closed system which eliminates most of the fumes. Resin infusion processes can also infuse resin into a preform with a vacuum thus avoiding air bubble entrapment. Resin infusion methods allow for better control over part dimensions and fiber volume fraction. Thus, resin infusion methods overcome many of the limitations of wet lay-up processes. Unfortunately, until recently resin infusion processes tended be more expensive than wet lay-up because of the more expensive molding apparatus required. In the past, a rigid closed mold was required. Even though closed mold methods are improvements over wet lay-up methods, there are problems with closed mold methods beyond that of cost. When infiltrating high volume fraction preforms, standard closed mold methods require high resin injection pressures and long infiltration times because the low permeability of high volume fraction preforms.
More recently, several novel vacuum infusion techniques were introduced which do not require an expensive closed mold. Instead, they incorporate a single rigid mold surface upon which the preform rests. The preform is covered by a impermeable sheet and is sealed at its periphery forming a preform cavity which can be evacuated using a vacuum pump. Atmospheric pressure provides both the compaction force on the preform and also the driving force for resin infusion from an external supply into the lower pressure preform cavity. Despite the simplicity of this approach there are still problems with the infusion process because many high viscosity resins cannot adequately infiltrate into low permeability preforms.
The resistance to resin infiltration increases with the distance the resin must flow through the preform. Typically, in vacuum assisted resin infusion, the injection pressure cannot exceed the ambient atmospheric pressure without pressing the outer sheet away from the preform. Thus, infiltration is often slow and incomplete. Often, regions of the preform are not infiltrated with resin, while excess resin collects in undesired locations.
One clever way to overcome the problem of a low permeability preform is to artificially increase the permeability at certain locations within the preform cavity. There have been several embodiments of this fundamental concept in the art. These include but are not limited to: placement of tubular arteries between adjacent preform layers, drilling an array of holes in a rigid plate which rests on the preform, and the use of a distribution medium on top of the preform. All these methods have their own limitations. Of these methods, the processes incorporating a distribution medium are the most useful. They are also the most closely related methods to those disclosed in the present patent and therefore will be described in greater detail.
In the distribution medium process, a preform is placed on a tool surface and covered with a permeable sheet. A distribution medium is placed on top of the permeable sheet and is covered with a continuous non-permeable sheet sealed at its periphery. A vacuum is drawn on the entire assembly of preform, permeable sheet, and distribution medium. Resin is introduced to the distribution medium which provides high permeability pathways for the resin to distribute itself over the entire top surface of the preform. The injection pressure, which must be less than atmospheric pressure, forces most of the resin through the permeable membrane and into the preform. Satisfactory resin impregnation is usually achieved since there is a relatively small resistance to infiltration of the preform in the thickness direction in comparison with the in-plane direction. The lower resistance to infiltration also results in shorter infusion times.
Despite the benefits of this improved resin infusion method, it still has some disadvantages which make it unattractive for many applications. When resin comes into contact with the distribution medium it remains trapped in the distribution medium and is subsequently cured. After the resin has cured, the permeable membrane, the clogged distribution medium, and the impermeable outer membrane must be separated from the composite part and discarded as waste. While the process is quite useful for proto-typing and low volume production, the amount of waste produced is incompatible with the required efficiencies of mass production.
These and other problems remain in the field of infusion molding of composite structures. In this patent we describe novel means of overcoming the aforementioned difficulties.